1. Field of the Invention
The present invention relates to a polymer material which has excellent fire-retardancy, and emits less corrosive gas and less smoke on burning.
2. Description of the Related Art
Polymer materials are desired to be fire-retardant to prevent fire accident or fire spreading in use for insulating materials such as electric wires and cables; sheath materials; enclosures and internal parts of electric, electronic, and office automation apparatuses; interior materials of vehicles; building materials, and so forth. Many polymer materials for such uses are enforced to be fire retardant by legislation. For fire retardancy of polymer materials, known fire-retardant additives include halogen type fire-retardant additives, magnesium hydroxide, aluminum hydrozide, red phosphorus, and phosphorus compounds. These fire-retardant materials, however, are not perfect, and have disadvantages below.
The halogen type fire-retardant additives, which give a high level of fire retardancy (for example, UL-94V-0, V-1, or V-2) by addition in a small amount, generate soot or smoke in a large amount on burning. Further, the halogen type fire-retardant additives emit more or less acidic substances such as a hydrogen halide by heat of processing or at the time of fire accident, which would cause corrosion of the machines for resin processing, or produce adverse effects on human health or apparatuses in the vicinity of a fire site.
Metal hydroxides as the fire retardant, such as magnesium hydroxide and aluminum hydroxide, are required to be added to the resin in a larger amount, although they emit neither smoke nor corrosive gas. The addition thereof in a larger amount will impair the mechanical strength, light-weight, and other favorable characteristics of the polymer.
The phosphorus type fire-retardant additives such as red phosphorus and phosphoric acid esters are effective in a small amount for polyamides, polyesters, polyphenylene oxides and other engineering plastics. However, they are less effective for fire retardation of general purpose polymers such as polyolefins and polystyrenes.
Therefore, a fire-retardant additive is demanded which contains no halogen, emits less smoke and less corrosive gas, and is effective in a smaller amount of addition. Promising techniques therefor have been disclosed in which heat-expandable graphite and a synergist are used in combination. For example, Japanese Patent Laid-Open Publication 6-73251 discloses fire-retardation of polystyrene by addition of a small amount of combination of red phosphorus and heat-expandable graphite. Japanese Patent Laid-Open Publication 6-25476 discloses fire-retardation of polyolefin by addition of a small amount of combination of red phosphorus or a phosphorus compound with heat-expandable graphite.
However, the red phosphorus itself is a substance under control as a dangerous object by Japanese Fire Protection Law, and is not necessarily easily handleable in storage, transportation, and blending with a polymer. Most of phosphorus compounds are low-melting, and is not necessarily easily blendable with a polymer.
The present invention has been accomplished to solve the above problems, and intends to provide a fire-retarding technique for a resin by use of a combination of heat-expandable graphite and a novel synergistic material without using red phosphorus or a phosphorus compound.